Interference reducing radio receiver



Nov. 30', 1948,

G. LEHMANN INTERFERENCE REDUCING RADIO RECEIVER Filed April 13, 1943 L F L .h.

ATTO

Patented Nov. 30, 1948 UNITED STATES TENT Q FFICE.

INTERFERENCE REDUCING RADIO RECEIVER Application April 13, 1943, Serial No. 482,909 In France August 30, I941 v Claims. 1

This invention. deals with radio receiving systems and particularly with means for minimizing the effects of interference such as is caused particularly by ignition systems of internal combustion engines.

An objectof this invention is to provide an anti-interference arrangement for radio receivers which is particularly effective against short.

and intense interfering electrical disturbances.

According to a feature of this invention the first or high frequency amplifier of an amplitude modulated wave receiver is adapted to pass a broad band width of radio frequency energy and is separated from the following or second amplifier, that is the audio frequency amplifier, which is adapted to pass a narrow band of audio frequencies, by an amplitude limiting device.

In accordance with another feature of this invention it is possible by the simple expedient of substituting a frequency discriminator fer the detector of the system to obtain the full advantages of this invention for a frequency modulated wave receiver.

A further and more detailed disclosure of the invention will be given in. the following description in connection with the attached drawings;

This invention resides substantially in the combination, construction, arrangement and relative location of parts, all as will be described in. detail below.

In the accompanying drawings,

Figure l is a diagrammatic illustration of the subject matter of this invention as applied to amplitude modulated systems; and

Figure 2. is a similar illustration of the application of the invention to frequency modulated systems.

Many arrangements have been devised and proposed for eliminating interference in the reception of radiowaves. This invention is concerned with the. same problem, but provides in a very simple manner the insurance of eflicient protection. against short and intense interference. This protection is obtained in a substantial manner in both amplitude and frequency modulated receiving systems.

If we call 11 the highest audio frequency to be. transmitted the conventional amplitude modulation receivers comprise a high frequency-and an intermediate frequency amplifier designed to have a band width 211 followed by a low' frequency amplifier designed to have a band width (1. In the. system herein disclosed the high frequency and intermediate frequency amplifiers,

and if desired the first stage of the. audio amplifier, are designed to have an excessive band width corresponding to an arbitrarily chosen audio frequencyD, that is to have a band width of 2D. The frequency D is preferably as large as possible, the band width 2D being limited only by the separation of the channels above and below the carrier frequency of the wave to be received and the amount of random noise which can be tolerated, since the increased band width increases the random noise.

The receiver then comprises one link o-r'portion called the wide band link transmitting a maximum frequency D and another link called the narrow'bandlink transmitting a maximum frequency d. v

Further, in accordance with this invention an amplitude limiting device of any suitable well known type, as for example the type used in frequency modulated receivers is interposed between these two link-s.

This subject matter is diagrammatically illustrated in Figure 1, wherein the entire receiver is shown comprising a high frequency amplifier I, an intermediate frequency amplifier 2., an amplitude limiting device 3, a detector 4 which may also be called a demodulator-detector, and an audio frequency amplifier 5. Each of these devices is individually old and well known. in the art, and most commonly comprise multi.-e1ectrode vacuum tubes and associated circuits to provide the functions indicated. The circuits of the high frequency and intermediate frequency portions are designed to pass a band width 2D andthe circuitof the audio frequency amplifier is designedto pass a band width (1. It follows thatthe limiting device 3' and the detector 4 will have circuits'designedto pass bands of 2D and D respectively.

The purpose of. this combination will now be explained. It is; assumed that the maximum amplitude of output currents passed by the limiter can be represented. by L. The receiver is adjusted in. such a manner that the peak amplitude of the useful signal does not exceed the value L. Because of this precaution the limiter does not change the amplitude modulated signal.

It will be shown that for such a receiver, if the. peak amplitude of the useful signal can reach the value L, the peak amplitude of. interference at the output of the receiver cannot exceed thevalue As a consequence the signal to noise ratio will always remain larger than the value of In a conventional receiver peak interference can reach very high values very much higher than the signal itself and the signalto noise ratio can therefore become much smaller than unity.

The useful signal whose amplitude must not exceed the maximum amplitude of the limiter goes through the receiver without alteration as it would through a conventional receiver. On the other hand sudden and sharp interferences are found at the output of the limiter only under the following conditions (1)' their amplitude is limited to the value L, (2) their duration is approximately equal to the time constant t of the broad band link. This time constant it being short, interferences are therefore of short duration. Interferences of limitedamplitude L and of duration t corresponding to a frequency band D; go through the narrow band link of band width 11. It is a well known fact that the peak amplitude of an impulse at the output of a filter is directly proportional to the band width of the filter and the time duration of the impulse is inversely proportional to the band width of the filter. The impulse passing through the narrow band amplifier of band width ,d and time constant It will therefore be reduced in amplitude and increased in time duration. Since the time duration of the impulse of L amplitude was t when it entered the amplifier 5 and t when it emerged from the amplifier, the amplitude will be equal to t LX27 But, since the time constant of a filter is inversely proportional to its band width,

t will equal which can be substituted into the last equation. As a result the amplitude of interference at the The effect on the noise impulse is indicatedon the wave diagrams immediately above Fig. 1. The wave diagrams illustrating the output of the IFamplifier 2 and the limiter 3 are indications of one-half. the envelope of the modulated carrier wave, while that indicating the output of the audio amplifier 5 is the actual envelope which would be produced at that point.

- The gain obtained by the present system may reach values of the order of 20 to 40 db., and even more whenever the separation between the frequencies of the transmitters is wide enough to permit the use of a receiver having low selectivity. It may be seen that. the gain will be higher when selectivity is lower.

In particular, and as an example in aviation telephone communicating systems the spacing between transmitters is always selected to be more than 50 kc., the highest signal frequency transmitted being of the order of 4 kc. We therefore have D=25 kc. and 11:4 kc. With such values and employing this invention peak interference values of one-sixth the peak amplitude of the signal represent the worst condition of interference.

Since the effect of motor ignition noise is a very important factorin aircraft radio communication, such a signal to noise ratio is most advantageous. In fact the use of superheterodyne receivers in airplanes had to be given up because of their extreme sensitivity to interference and superregenerative receivers had to be used having a low sensitivity to interference, but whose operation is often unstable. In ground stations superheterodyne receivers are more frequently used but these receivers are very sensitive to interference from internal combustion engines. However, by employing the subject matter of this invention without modification of the transmitters in use, it is possible to use superheterodyne receivers both in the airplanes and on the ground which are practically insensitive to ignition noises.

The present invention is equally applicable to frequency modulated receivers by simply substituting for the detector 4 a discriminator 6 as illustrated in Figure 2. The details of construction of suitable discriminators for this purpose are well known in the art and need no further disclosure in order to practice this invention.

It follows, therefore, that this invention is of great interest and value to builders of anti-interference receivers for either amplitude or frequency modulated systems because equal protection can be given in the two systems against short and intense interference disturbances. It is easily possible to convert either system to the other by the simple interchange of the discriminator for the detector, or vice versa.

For emphasis it is again noted that a particular advantage of the present invention is that it provides a completely insensitive receiver with respect to ignition noises from internal combustion engines for amplitude modulated transmissions without requiring the modification of the transmitters as now presently used, as well as when it is desired to receive frequency modulated transmissions.

From the above description it will be apparent to those skilled in the art that the subject matter of this invention is capable of some variation by those skilled in the art, and I do not therefore desire to be strictly limited by the disclosure, but rather by the claims.

What is claimed is:

1. In a radio receiver, the combination of a high frequency amplifier having a wide frequency band width, with respect to the maximum signal frequency band transmitted, a further amplifier stage having a frequency band width equal to the maximum signal-frequency to be transmitted, and coupling means comprising an amplitude limiting device and passing the frequency band Width of said high frequency amplifier, interconnecting said amplifiers, the said amplifier having a band width equal to the maximum signal-frequency being constituted by a stage of audio-frequency amplification.

2. A radio receiver according to claim 1, in which said coupling mean-s comprises also a demodulator-detector which has a band width substantially equal to that of said high frequency amplifier.

3. In a radio receiving system the combination including a high frequency amplifier characterized in that it passes a frequency band greater than the band corresponding to the maximum signal frequency transmitted, an amplitude limiting device connected to the output of said amplifier, a demodulator-detector connected to the output of said limiting device, and an audio-frequency amplifier connected to the output of said demodulator-detector designed to pass a frequency band equal to the band corresponding to the highest signal frequency to be transmitted, said demodulator-detector being designed to pass a band width equal to that of the high frequency amplifier.

4. A radio receiving system according to claim 3, in which the demodulator-detector comprises a frequency discriminator.

5. In a radio receiving system, the combination including a high frequency amplifier comprising at least one frequency conversion stage, said amplifier characterized in that it passes a frequency band greater than the band corresponding to the maximum signal frequency transmitted, an amplitude limiting device connected to the output of said amplifier, a demodulatordetector connected to the output of said limiting device, and an audio-frequency amplifier connected to the output of said demodulator-detector designed to pass a frequency band equal to the band corresponding to maximum signal frequency to be transmitted, said limiting device and said demodulator-detector being designed to pass a frequency band of width corresponding to that of the high frequency amplifier.

GERARD J. LEHMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 734,048 Ehret July 21, 1903 2,087,288 Landon July 20, 1937 2,099,311 Nicholson, Jr Nov. 16, 1937 2,116,501 Armstrong May 10, 1938 2,116,502 Armstrong May 10, 1938 2,151,739 Burill Mar. 28, 1939 2,180,355 I-Iaffcke Nov. 21, 1939 2,192,189 I-Iaffcke Mar. 5, 1940 2,200,036 Mountjoy May 7, 1940 2,203,465 Landon June 4, 1940 2,248,267 Bacon 1 July 8, 1941 2,261,643 Brown Nov. 4, 1941 2,266,713 Meier Dec. 16, 1941 2,273,098 Foster Feb. 17, 1942 2,295,323 Armstrong Sept. 8, 1942 Re. 22,302 Case Apr. 20, 1943 

